High-energy Wave Events Research Articles

Headland Rip Very Low Frequency Fluctuations and Surfzone Eddies During High-Energy Wave Events

Abstract A wave-group-resolving model is used to investigate the driving mechanisms and the spatiotemporal variability of very low frequency (VLF) fluctuations of a headland deflection rip, measured during a 4-m oblique wave event. Surfzone eddies (SZE) occurring in the presence of a strongly sheared longshore current V at a longshore-uniform beach are first modeled. The spectral signature and the variability of SZE are displayed and compared with the literature. The model is then used to explore the dynamics of vorticity in the surf zone and against a headland under energetic oblique wave conditions. The resulting weakly sheared V is found to host large-scale SZE propagating toward the headland at a speed decreasing seaward. Vorticity animations and spectral diagrams indicate that VLF fluctuations of the deflection rip are driven by the deflection of the upstream SZE. In line with measurements, periods from 40 min to 1 h dominate the spectrum hundreds of meters from the headland at low tide. At high tide, vorticity spectra in the rip are much narrower than in the surf zone, suggesting that the headland enforces the merging of SZE. This mechanism is further analyzed using idealized simulations with varying headland lengths, aiming at extending traditional deflection patterns at the VLF scale. Finally, we discuss the existence of a continuum in SZE driving mechanisms, going from fully wave-group-driven to both wave-group- and shear-instability-driven SZE for weakly and strongly sheared V, respectively. This continuum suggests the importance of wave groups to produce SZE under energetic wave conditions.

Applying a rule-based object-based image analysis approach for nearshore bar identification and characterization

Nearshore bars are important coastal features that influence beach–dune dynamics and protect the coast from high-energy wave events. Historically, in situ methods were used to study beach–dune dynamics and nearshore bars. Video monitoring systems, such as Argus, have been the favored remote sensing approach for studying nearshore bars dynamics and evolution. These systems are costly to install and maintain, and a limited number of stations are available. Our study introduces a lower cost alternative to study these coastal features. A rule-based classification object-based image analysis approach is introduced to identify nearshore bars (and their characteristics, such as bar length, width, count, and distance from the wet/dry line) from very-high-resolution (VHR) multispectral imagery. Nearshore bars were successfully identified and characterized as single and multiple bar systems. Our study demonstrates that VHR satellite remote sensing is a viable alternative with comparative results to video monitoring systems. Their capability of acquiring high-resolution imagery in large spatial extent is superior in continuous observation and characterization of nearshore bars. The proposed approach provides information at a more relevant spatiotemporal scale at which these systems evolve, which is helpful to coastal scientists and managers.

The place of courage

I have been rereading my editorial for 57(2) and cannot quite comprehend how quickly the quarter has flown by. Electing to leave off writing this present editorial until the end of the Institute of Australian Geographers 49th annual conference, held last week in Hobart, I recall sitting in Washington DC in April, thinking about the Hobart meeting and about conference culture more generally. Now, suddenly, the Hobart meeting is over and, as I write, people have wound or are winding their ways home. For those of us who reside here, of course, that journey is blissfully short—a rare event for us, and, I confess, welcome. So today, as I gaze at Kunanyi’s snow-covered peaks and valleys, I have time to reflect on the conference in a way more leisurely than might be the case had I been travelling. Scanning the contents for this issue, readers will see we have been fortunate to reproduce a speech given at the conference opening by University of Tasmania’s Vice Chancellor and President, Professor Rufus Black. I will not pre-empt what he writes except to note that he was gracious and authentic in his engagement with us and encouraged us to continue to be courageous in our scholarly considerations of place, in our defence of place, and in the example we set for others in relation to place. Readers will also see that we have included a few reflections on the conference from the lead organisers, Jason Byrne and Aidan Davison. In addition, Carolyn Finney has graciously given her permission to reproduce the moving poem she read during her keynote address at the conference.

Boulder emplacement and remobilisation by cyclone and submarine landslide tsunami waves near Suva City, Fiji

The characteristics of a reef-top boulder field created by a local submarine landslide tsunami are presented for the first time. Our examination of large reef-derived boulders deposited by the 1953 tsunami near Suva City, Fiji, revealed that shorter-than-normal-period tsunami waves generated by submarine landslides can create a boulder field resembling a storm boulder field due to relatively short boulder transport distances. The boulder-inferred 1953 tsunami flow velocity is estimated at over 9ms−1 at the reef edge. Subsequent events, for example Cyclone Kina (1993), appear to have remobilised some large boulders. While prior research has demonstrated headward retreat of Suva Canyon in response to the repeated occurrence of earthquakes over the past few millennia, our results highlight the lingering vulnerability of the Fijian coastlines to high-energy waves generated both in the presence (tsunami) and absence (storm) of submarine failures and/or earthquakes. To explain the age discrepancies of U-Th dated coral comprising the deposited boulders, we introduce a conceptual model showing the role of repeated episodes of tsunamigenic submarine landslides in removing reef front sections through collapse. Subsequent high-energy wave events transport boulders from exposed older sections of the reef front onto the reef where they are deposited as ‘new’ boulders, alongside freshly detached sections of the living reef. In similar situations where anachronistic deposits complicate the deposition signal, age-dating of the coral boulders should not be used as a proxy for determining the timing of the submarine landslides or the tsunamis that generated them.

ENHANCED FIELD OBSERVATION BASED PHYSICAL AND NUMERICAL MODELLING OF TSUNAMI INDUCED BOULDER TRANSPORT PHASE 1: PHYSICAL EXPERIMENTS

Coasts around the world are affected by high-energy wave events like storm surges or tsunamis. By focusing on tsunami impacts, we investigate tsunami-induced transport of boulders by an interdisciplinary combination of field observations, laboratory experiments and advanced numerical modelling. In phase 1 of the project we conduct physical laboratory experiments based on real-world data. Following the experimental phase we will develop an enhanced numerical boulder transport model (BTM) based on an existing two-phase model.

Storm effects on intertidal invertebrates: increased beta diversity of few individuals and species.

Climate change is predicted to lead to more extreme weather events, including changes to storm frequency, intensity and location. Yet the ecological responses to storms are incompletely understood for sandy shorelines, the globe’s longest land-ocean interface. Here we document how storms of different magnitude impacted the invertebrate assemblages on a tidal flat in Brazil. We specifically tested the relationships between wave energy and spatial heterogeneity, both for habitat properties (habitat heterogeneity) and fauna (β-diversity), predicting that larger storms redistribute sediments and hence lead to spatially less variable faunal assemblages. The sediment matrix tended to become less heterogeneous across the flat after high-energy wave events, whereas β-diversity increased after storms. This higher β-diversity was primarily driven by species losses. Significantly fewer species at a significantly lower density occurred within days to weeks after storms. Negative density and biomass responses to storm events were most prominent in crustaceans. Invertebrate assemblages appeared to recover within a short time (weeks to months) after storms, highlighting that most species typical of sedimentary shorelines are, to some degree, resilient to short-term changes in wave energy. Given that storm frequency and intensity are predicted to change in the coming decades, identifying properties that determine resilience and recovery of ecosystems constitute a research priority for sedimentary shorelines and beyond.

Nearshore hydrodynamics, beachface cobble transport and morphodynamics on a Pacific atoll motu

Measurements of wave transformation, beachface cobble transport and shoreline morphology change were undertaken over a 14month period at Fatato Island, a cobble and sand ‘motu’ on the southeast rim of Funafuti Atoll, Tuvalu. Results document significant cobble transport and beach profile change on Fatato during a year in the absence of any significant high energy wave events. Changes in beachface volume were measured on 11 across-shore beach profiles, and show marked variations of +7.1 and −7.01m3 between profiles and between survey periods. Net volumetric beachface change over the 14month experiment was approximately zero, indicating a balance between erosion and deposition and net conservation of sediment volume on the ocean shoreline. Cobble movement was measured using Passive Integrated Transponder (PIT) tags inserted in 300 randomly selected clasts (mean b-axis=75mm) on the mid-beachface. Tagged particles were re-surveyed after 6 and 14months, achieving 76% recovery, and identify large excursions of clasts on the beachface and dominant southwest alongshore transport, with an average net movement of 68m after 14months. A simple agent based probabilistic model was developed to simulate cobble movement and explore the coupling between hydrodynamic and shoreline processes. Model simulations reveal: predominant south-westerly cobble transport is driven by mean wave conditions associated with easterly trade winds, and; long-period swell events that approach the reef from the south drive episodic north-easterly transport reversals and significant onshore transport that can strand particles on the upper beach. Results present the first observations of morphodynamic behaviour of an atoll motu and highlight the highly dynamic nature of motu shorelines under mean energy conditions. Such results challenge conventional perspectives that motu are storm adjusted landforms that require extreme events to stimulate geomorphic change and inform future projections of island change.

Multiple dating approach (14C, 230Th/U and 36Cl) of tsunami-transported reef-top boulders on Bonaire (Leeward Antilles) – Current achievements and challenges

Dating the transport/deposition time of supratidal coarse-clast deposits is difficult, limiting their value for inferring frequency-magnitude patterns of high-energy wave events. On Bonaire (Leeward Antilles, Caribbean), these deposits form prominent landforms, and transport by one or several Holocene tsunamis is assumed at least for the largest clasts. Although a large dataset of 14C and electron spin resonance (ESR) ages is available for major coral rubble ridges and ramparts, it is still debated whether these data reflect the timing of major events, and how these datasets are biased by the reworking of coral fragments. As an attempt to overcome the current challenges for dating the dislocation of singular boulders, three distinct dating methods are implemented and compared: (i) 14C dating of boring bivalves attached to the boulders; (ii) 230Th/U dating of post-depositional, secondary calcite flowstone and subaerial microbialites at the underside of the boulders; and (iii) surface exposure dating of overturned boulders via 36Cl concentration measurements in corals. Approaches (ii) and (iii) have never been applied to coastal boulder deposits so far. The three 14C age estimates are older than 40ka, i.e. most probably beyond the applicability of the method, which is attributed to post-depositional diagenetic processes, shedding doubt on the usefulness of this method in the local context. The remarkably convergent 230Th/U ages, all pointing to the Late Holocene period (1.0–1.6ka), are minimum ages for the transport event(s). The microbialite sample yields an age of 1.23±0.23ka and both flowstone samples are in stratigraphic order: the older (onset of carbonate precipitation) and younger flowstone layers yield ages of 1.59±0.03 and 1.23±0.03ka, respectively. Four coral samples collected from the topside of overturned boulders yielded similar 36Cl concentration measurements. However, the computed ages are affected by large uncertainties, mostly due to the high natural chlorine concentration. After correction for the inherited component and chemical denudation since platform emergence (inducing additional uncertainty), the calculated 36Cl ages cluster between 2.5±1.3 and 3.0±1.4ka for three of four boulders whilst the fourth one yields an age of 6.1±1.8ka, probably related to a higher inheritance. These 230Th/U and 36Cl age estimates are coherent with a suggested tsunami age of <3.3ka obtained from the investigation of allochthonous shell horizons in sediment cores of northwestern Bonaire. Whilst 230Th/U dating of post-depositional calcite flowstone appears to be the most robust and/or accurate approach, these results illustrate the potential and current limitations of the applied methods for dating the dislocation of supralittoral boulders in carbonate-reef settings.

The initial morphological response of the Sand Engine: A process-based modelling study

Sand nourishments are presently widely applied to maintain or enhance coastal safety and beach width. Over the last decades, global sand nourishment volumes have increased greatly, and the demand for nourishments is anticipated to increase further in coming decades due to sea level rise. With the increase in nourishment size and the request for more complex nourishment shapes, an adequate prediction of the morphodynamic evolution is of major importance. Yet, neither the skill of current state-of-the-art models for such predictions nor the primary drivers that control the evolution are known. This article presents the results of a detailed numerical modelling study undertaken to examine the model skill and the processes governing the initial morphological response of the Sand Engine and the adjacent coastline. The process-based model Delft3D is used to hindcast the first year after completion of the mega-nourishment. The model reproduces measured water levels, velocities and nearshore waves well. The prediction of the morphological evolution is consistent with the measured evolution during the study period, with Brier Skill Scores in the ‘Excellent’ range. The model results clearly indicate that the sand eroded from the main peninsular section of the Sand Engine is deposited along adjacent north and south coastlines, accreting up to 6km of coastline within just one year. Analysis of model results further show that the erosional behaviour of the Sand Engine is linearly dependent on the cumulative wave energy of individual high energy wave events, with the duration of a storm event being more dominant than the maximum wave height occurring during the storm. The integrated erosion volume due to the 12 events with the highest cumulative wave energy density accounts for about 60% of the total eroded volume of the peninsula, indicating that the less energetic wave events, with a higher probability of occurrence, are also important for the initial response of the Sand Engine. A structured model experiment using the verified Delft3D model indicates that wave forcing dominates the initial morphological response of the Sand Engine, accounting for approximately 75% of the total erosion volume in the first year. The vertical tide is the second most important factor accounting for nearly 17% of the total erosion volume, with surge, wind and horizontal tide playing only a minor role.

The extreme 2013/2014 winter storms: Beach recovery along the southwest coast of England

Sand and gravel beaches naturally act as a coastal buffer, absorbing wave energy and dynamically adapting to the seasonal and long-term wave climate. Significant shifts in nearshore morphology can occur during extreme wave events, which can have a significant impact on coastal vulnerability. During the winter of 2013/14, the Atlantic coast of Europe received an unprecedented sequence of very energetic wave conditions (8-week mean offshore Hs=4.4m). These events caused extensive physical (beach and dune erosion) and socio-economic (flooding, damage to infrastructure) impacts throughout the west coast of Europe. Many monitored sites in the UK and France were in their most eroded state since morphological records began (5–10years). We consider the geomorphological significance of the storm response at 38 natural beaches in the southwest of England, ranging from semi-sheltered reflective gravel barriers to ultra-dissipative exposed sand beaches with dunes. The extent and patterns of post-storm recovery are examined in detail at three beaches with characteristic storm response behaviours. Exposed sandy beaches were dominated by cross-shore transport processes leading to significant loss of sediment offshore from the intertidal zone (>200m3/m); exposed gravel beaches were dominated by overwash with significant loss landward; and semi-sheltered sites exposed to more oblique wave forcing were dominated by a rotational response due to alongshore sediment redistribution. Due to these contrasting responses, mechanisms and timescales for beach recovery displayed strong inter-site and intra-site variations. In offshore and rotational cases, the recovery processes were multi-annual, comprising seasonal to decadal signals and were intrinsically linked to the storm response mechanisms, while permanent losses occurred when overwash dominated. We show that post-storm recovery does not necessarily occur during calm periods and that in many cases high-energy wave events appear to be essential for recovery of sediment (offshore and counter-rotation). Our results highlight the significance of dominant climatic oscillations, multi-annual storm sequencing, storm tracks and resultant variations in wave angle, in controlling the impact that extreme wave events have on contrasting sand/gravel beaches in exposed/sheltered locations.

Temple-complex post-dates tsunami deposits found in the ancient harbour basin of Ostia (Rome, Italy)

Detailed geophysical and geoarchaeological investigations carried out in Ostia, ancient harbour of Rome, revealed two different generations of harbour basins and also proved the repeated impact of high-energy wave events on the study area. West of Ostia, at the southern bank of the Tiber, a lagoonal harbour existed from the 4th and 2nd cent. BC but was affected by strong siltation. At the same site, a river harbour was subsequently established from the 1st cent. AD onwards. Fluvial deposits of medieval age finally document Tiber river bank erosion affecting the abandoned site. Within the sedimentary record, distinct high-energy event deposits were found and seem to be related to tsunami impact that hit the ancient harbour site. Event I occurred prior to the harbour foundation between the 8th and 5th cent. BC and induced significant environmental changes. Event II hit the lagoonal harbour most probably in the 4th cent. BC. Event III caused a widespread burial of the lagoonal harbour basin which, at that time, was already silted up. Built directly on top of the youngest event deposit found at Ostia, the foundation of the (navalia-) temple-complex post-dates tsunami event III to the time before the early 1st cent. AD. Subsequently, the site was used as a river harbour.

Advantages of beachrock slabs for interpreting high-energy wave transport: Evidence from Ludao Island in south-eastern Taiwan

Powerful typhoons frequently affect eastern Taiwan. On Dabaisha beach on the SW coast of Ludao Island, a small volcanic island lying SE off mainland Taiwan, the presence of numerous beachrock slabs provides evidence of the sediment transport capabilities of high-energy waves generated by these storms. With a well-defined beachrock source from which the slabs were derived, it is easy to differentiate clasts transported in run-up or backwash flow based on their current position relative to the outcrop. In order to estimate the wave-generated flow velocities from the measured beachrock slabs, the hydrodynamic equations for boulder transport by Nandasena et al. (2011) have been rearranged so that they are appropriate for this particular geomorphic setting, where exposed in situ beachrock outcrops liberate clasts which may then be transported either by run-up onto the beach or backwash flow onto adjacent reef platforms. Such settings are common on tropical coastlines with fringing coral reefs and beaches comprising mostly biogenic sediments. In the Dabaisha beach case, the lowest flow velocities required to transport all measured beachrock slabs from their source outcrop to current positions are 2.18–3.16m/s for run-up and 1.76–3.24m/s for backwash. With a discussion of advantages and limitations on the use of beachrock clasts for interpreting wave behaviour, this paper presents another proxy for studying high-energy wave events that is highly suitable for certain carbonate-rich coastlines.

Wind-driven waves in a shallow estuarine lake with muddy substrates: St Lucia, South Africa

Zikhali, V., Tirok, K., Stretch, D., 2014. Wind-driven waves in a shallow estuarine lake with muddy substrates: St Lucia, South Africa. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 729-735, ISSN 0749-0208.Wind-waves in shallow lakes or estuaries with muddy substrates can drive sediment re-suspension and cause high turbidity levels that can negatively impact the productivity of photosynthetic organisms. This investigation evaluates the efficacy of a simple semi-empirical model (Young and Verhagen, 1996 : Coastal Engineering, 29, 47–78) for predicting the wave characteristics in these systems in order to include their effects in ecosystem models. The southern basin of the St Lucia estuarine lake in South Africa was used for a case study. Average depths are about 1 m with fetches up to approximately 10 km. Substrate materials vary from sandy to muddy with deeper locations predominantly the latter. An array of pressure sensing wave poles was deployed to measure significant wave heights and periods to compare with model predictions. The influence of the wind speed, fetch, fetch-averaged depth, and substrate composition were evaluated. Most of the observed waves were fetch limited during the conditions that prevailed during the two field trips. The results indicate that the model adequately captures the high energy wave events for persistent wind speeds and directions, but that there is considerable variability in its performance generally. Some of this variability can be attributed to difficulties in estimating appropriate fetch and depth parameters for variable winds and in the context of a lake with compound shape and variable bathymetry. There was no clear evidence of significant wave attenuation due to the muddy substrates.

Bonaire's boulder fields revisited: evidence for Holocene tsunami impact on the Leeward Antilles

Supralittoral boulders and blocks are prominent sedimentary features along rocky shorelines worldwide. In most cases, their deposition is attributed to high-energy wave events (tsunamis, severe storms). Even though tsunami waves are expected to have higher transport capacities compared to storm waves, megaclasts of up to 100 t were observed to have been moved laterally by the latter waveform. The deduction of certain extreme wave events (tsunamis, severe storms) from the boulder record thus remains a major challenge in palaeo-event research; the debate on their differentiation is ongoing. At the eastern coast of Bonaire (Leeward Antilles) in the southern Caribbean, numerous limestone blocks and boulders (up to c. 130 t) are distributed on top of a 3–6 m a.s.l. (above mean sea level) palaeo-reef platform. Disagreement exists among a number of authors concerning the transport processes involved in the formation of the boulder fields. In this paper, state-of-the-art modelling approaches of coastal boulder entrainment and transport were applied in order to provide new and more reliable data to support/challenge the working hypothesis of tsunami deposition. To improve the reliability of the boulder transport model, more realistic input parameters were provided by new DGPS measurements of the boulder dimensions and the calculation of bulk densities by taking into account the heterogeneity of the reef-rock boulders. Existing hydrodynamic equations were modified to allow for the irregular shape and real dimensions of the boulders. The results indicate that (i) boulder weight and dimension, and thus (ii) calculated wave energy and wave heights were overestimated in most of the previous studies, where calculations of boulder volume were based on multiplication of the main axes. The results of this study and wave heights observed during recent severe tropical cyclones seem to rule out storm-generated waves for the dislocation of the largest blocks. However, the majority of coarse-clast deposits may have been generated by periodic hurricane swells. The results underline the significance of more realistic field data in modelling boulder transports by waves.

Video observations of nearshore bar behaviour. Part 2: alongshore non-uniform variability

A considerable portion of the variability in nearshore sandbars is related to changes in the plan shape of quasi-rhythmic alongshore non-uniform features, such as rip channels and crescentic shapes. These changes may include changes in their alongshore length, cross-shore amplitude and alongshore position. Here, we use complex empirical orthogonal eigenfunction analysis to quantify these changes from a 3.4-year data set of almost daily time-exposure images of the double-barred coast at Noordwijk (Netherlands). The observed alongshore non-uniform features had alongshore lengths between 380 and approximately 3000 m and lifetimes in the order of months, considerably longer than the characteristic time scale of individual wave events. Transitions from one feature to another were mostly gradual, resulting from an alongshore differential growth in amplitude. Abrupt transitions, that is, the existing features disappeared entirely and were subsequently replaced by different features, were barely observed and did not always take place during high-energy wave events. The amplitude of the non-uniform features varied between 0 and 30 m on a weekly to monthly scale, unrelated to variations in the wave height. In addition, the features migrated back and forth along the shore with typical rates of O(10 m/ day) on weekly scales with the rates increasing with an increase in the alongshore component of the wave power. On the whole our observations suggest that alongshore non-uniform sandbar variability is governed by free behaviour rather than by the direct forcing of the prevailing wave conditions.

Beach and nearshore morphology of Lakes Manapouri and Te Anau, New Zealand: Natural models of the continental shelf

Abstract The foreshore and nearshore morphology of beaches around Lakes Manapouri and Te Anau, New Zealand, have developed a distinctive shelf form in response to lake wave processes. The characteristics of this shelf form vary systematically with the degree of exposure to wave activity. Shelf width ranges between 10⋅5 and 87⋅2 m and extends to depths of 1⋅2–5⋅8 m beneath mean lake level. The sediment distribution across the shelf reflects the shore normal dissipation of wave energy. Sandy bed load sediments fine lakeward to the outer edge of the shelf. Over the shelf edge, on the steep offshore slope, muddy sediments are dropped out of suspension. Wave climate studies show the outer edge of the shelf to be formed in response to infrequent, high energy wave events during periods of low lake levels when waves are highly effective in disturbing sandy sediments on the outer shelf. Parallels are drawn between the morphology and sediments on lacustrine shelves and prograding oceanic continental shelves.

Influence of turbidity currents upon basin waters : Emery, K. O., J. Hulsemann and K. S. Rodolfo 1962 Limnol. & oceanogr. 7(4): 439–446

Powerful typhoons frequently affect eastern Taiwan. On Dabaisha beach on the SW coast of Ludao Island, a small volcanic island lying SE off mainland Taiwan, the presence of numerous beachrock slabs provides evidence of the sediment transport capabilities of high-energy waves generated by these storms. With a well-defined beachrock source from which the slabs were derived, it is easy to differentiate clasts transported in run-up or backwash flow based on their current position relative to the outcrop. In order to estimate the wave-generated flow velocities from the measured beachrock slabs, the hydrodynamic equations for boulder transport by Nandasena et al. (2011) have been rearranged so that they are appropriate for this particular geomorphic setting, where exposed in situ beachrock outcrops liberate clasts which may then be transported either by run-up onto the beach or backwash flow onto adjacent reef platforms. Such settings are common on tropical coastlines with fringing coral reefs and beaches comprising mostly biogenic sediments. In the Dabaisha beach case, the lowest flow velocities required to transport all measured beachrock slabs from their source outcrop to current positions are 2.18–3.16 m/s for run-up and 1.76–3.24 m/s for backwash. With a discussion of advantages and limitations on the use of beachrock clasts for interpreting wave behaviour, this paper presents another proxy for studying high-energy wave events that is highly suitable for certain carbonate-rich coastlines.